Hydraulic tool systems typically employ multiple actuators provided with high-pressure fluid from a common pump. In order to efficiently accommodate the different flow and/or pressure requirements of the individual actuators, these systems generally include a pump having variable displacement. Based on individual and/or combined flow and pressure requirements, the pump changes a fluid displacement amount to meet demands. When demand is low, the displacement is reduced to conserve energy.
Typical variable displacement pumps used in hydraulic tool systems are known as axial plunger or swashplate-type pumps. This type of pump includes a plurality of plungers held against a plunger engagement surface of a tiltable swashplate. In most situations, the swashplate is generally planar and includes a smooth surface. A joint such as a ball and socket joint is disposed between each plunger and the engagement surface to allow for relative movement between the swashplate and the plungers. Each plunger is slidably disposed to reciprocate within an associated barrel as the plungers rotate relative to the tilted surface of the swashplate. As each plunger is retracted from the associated barrel, low pressure fluid is drawn into that barrel. When the plunger is forced back into the barrel by the plunger engagement surface of the swashplate, the plunger pushes the fluid from the barrel at an elevated pressure. In some situations, the tilt angle of the swashplate may be fixed such that the output of the pump is solely based on an input to the pump.
Another type of pump is known as a rotary plunger-type pump. This type of pump also includes a plurality of plungers held against a plunger engagement surface. In contrast to the swashplate-type pump, however, the plunger engagement surface of the rotary plunger-type pump is located directly on a stroke ring of the pump's rotating shaft, and the plungers are oriented at a right angle relative to the rotating shaft (i.e., rotary plunger-type pumps do not include a swashplate). The stroke ring is offset relative to a rotational axis of the shaft such that, as the shaft of the pump rotates, the stroke ring forces the plungers to reciprocate relative to their barrels, thereby pressurizing fluid similar to that described above.
An exemplary swashplate-type pump is disclosed in U.S. Pat. No. 5,644,949 (the '949 patent) issued to Murakami et al. on Jul. 8, 1997. In the pump design of the '949 patent, pairs of longitudinally aligned cylinder bores are formed in two separate cylinder blocks, and are arranged about a shaft at equal angular intervals. A double headed plunger is reciprocatingly disposed in each pair of cylinder bores. A wave cam is fixed to the drive shaft and rotates integrally with the shaft. The wave cam has front and rear cam surfaces, each having a two-cycle undulated displacement curve defined thereon. During operation, the plungers are caused to reciprocate within their respective bores by rotation of the wave cam and the shape of the displacement curve. In this manner, the plungers can be made to displace a refrigerant without having to tilt or control tilt of the wave cam.
While the pump of the '949 patent may be suitable for refrigerant purposes, it may lack applicability to other situations. Specifically, because the wave cam is fixed to the drive shaft, tilting of the wave cam to vary displacement of the plungers may not be possible. And, fixed displacement pumps may find limited application in, for example, machine hydraulic, lubrication, coolant, or fuel system arrangements. Further, the wave cam may induce undesired flow pulsations within the refrigerant flow. Lastly, the wave cam of the '949 patent may provide little benefit to a rotary plunger type pump
The disclosed pump is directed to overcoming one or more of the problems set forth above.